Enhanced oil recovery refers to the displacement of fluids from an injection well penetrating a crude-oil containing subterranean reservoir toward a production well penetrating the reservoir. Many processes and compositions have been developed for improving the percentage of crude oil that can be produced by this technique.
Aqueous micellar fluids have been developed for displacing crude oil through subterranean reservoirs. These fluids contain appropriate and sufficient amounts of surfactant for micelle formation in the aqueous fluid to increase the efficiency which these fluids will displace crude oil. Aqueous formation fluids can be connate water, the water remaining in the reservoir following a water-flood or water which is injected ahead of the micellar fluid to displace water from the reservoir. Sufficient micellar fluid is injected to produce a micellar bank of about 3 to about 25 percent (%) of the total pore volume of the portion of the reservoir from which crude oil is to be displaced. Following the micellar bank, an aqueous polymer-containing fluid can be injected into the reservoir to displace the micellar fluid. The polymer-containing fluid generally has a relatively low mobility through the reservoir to aid in the displacement of the crude oil. Sufficient polymer-containing fluid is injected to provide a polymer bank or mobility buffer bank of about 30 to about 100 percent of the total pore volume of the portion of the reservoir from which crude oil is to be displaced.
The fluids in the micellar bank and the polymer bank are generally formulated to contain inorganic salts. The nature and amount of the inorganic salts in the formulation depends in part on the mineral content of the reservoir and on the salinity of the aqueous fluids being displaced. In the formulation of the micellar fluid, the salinity thereof can be selected so that the interfacial tension between the micellar fluid and the crude oil is below about 100 millidynes/cm and preferably as low as about 10 millidynes/cm or lower. Low interfacial tension aids in the displacement of the crude oil through the reservoir. In formulating the fluids in the polymer bank, the salinity of the polymer-containing fluid can be selected to reduce the loss of surfactant in the micellar bank which can occur when the polymer bank mixes with the micellar bank. This loss of surfactant in the reservoir can be reduced by formulatng the polymer containing fluid at a low salinity.